It is well known that most lightning discharges are associated with predominantly negatively charged clouds. Two main categories of lightning strikes are encountered: Upward flashes from very tall structures and the more prevalent strikes associated with negative descending stepped leaders (ref. [1], [2]). The negative descending leader is surrounded with a negative space charge sheath which, as the negative leader approaches the ground, induces positive (image) charges on any grounded object in its sphere of influence. The higher the grounded structure and the nearer it is to the path of the descending negative leader, the more significant the induced charge on the grounded structure.
It is known that a lightning stroke current is a statistical variable that varies in a wide range from a few kA to a few hundred kA with a median of 25-35 kA. The attractive radius of a structure i.e. the maximum radial distance around the structure in which a descending leader would be captured by the structure increases with both the stroke current, which is associated with the negative space charge jacket and the structure height.
In recent years, based on progress in research on the physics of breakdown of long air gaps, our understanding of the mechanisms by which different ground structures are hit by lightning have been substantially improved. In particular the role played by the grounded object in the strike mechanism has been clarified. Modeling (ref. [6]) has shown that the attractive radius comprises two parts: a major part (two thirds or more) spanned by the positive leader emanating from the structure and the lesser part constituting the final jump between the negative and positive leader tips.
Electrostatic field analysis shows that the electric field enhancement at the surface of and in the vicinity of any grounded structure is predominately caused by the positive (image) charge that has been induced onto the grounded structure by the cloud charge and/or the descending negative leader and that this far exceeds the background field due to the cloud charge and/or the descending leader itself. Depending on the structural characteristics of the grounded object an inception field caused by the induced charge is reached when ionization of the surrounding air takes place causing corona discharge and positive streamer formation. Depending on the geometry of the grounded structure and the amount of induced positive charge the length of the positive streamer can grow into the meter range.
If the positive streamer reaches a critical size (ref. [3], [4]) a highly conducting stem is formed at the streamer junction to the structure and a positive leader is thereby formed. Contrary to the positive streamer which has a mean gradient of approximately 400-500 kV/m, the leader gradient is a function of both the leader current and the time duration of its existence. For a current of 1 A the leader gradient could be 30-50 kV/m i.e. approximately one tenth of the positive streamer gradient but for a leader current of the order of 100 A the leader gradient could go down to as low as 2-3 kV/m. This shows that contrary to the positive streamer, a positive leader is capable of traveling distances in the 100 m range without requiring unrealistically high electric potential.
It is important to note that not every positive leader emanating from a grounded structure will complete the trajectory to encounter the descending negative leader in a final jump. As the positive leader travels farther and farther from the structure its motion will be governed more and more by such parameters as space potential and the electric field ahead of the leader tip, which are determined more and more by the descending leader charge and less and less by the grounded structure. When conditions are not appropriate for continued propagation, the positive leader stops and the concerned grounded structure which started the positive streamer/positive leader process is not struck.
Objects that are struck by downward negative lightning are those which, due to their induced positive charge, “succeed” in creating long positive streamers resulting in the formation of a positive leader which progresses in a zone of increasing electric field in order to meet the approaching descending negative lightning leader in what is termed the final jump. The final jump takes place when the mean voltage gradient between the tip of the ascending positive leader and the tip of the descending negative lightning leader reaches 500-600 kV/m.
As seen from the negative descending lightning leader, all grounded objects with their respective induced positive charges are in a competition which determines: which among them will produce significant positive streamer activity and which among them will “succeed” in producing a positive leader that will complete the trajectory to the final jump. If no elevated structure “succeeds” in completing the trajectory to the final jump, the negative descending leader will proceed to ground by default. Therefore if the intent is to reduce the risk of such a lightning strike it will be of great advantage for any structure to remain electrically silent, i.e. to be inactive or inhibited in the game of producing long positive streamers.
The second type of lightning flash referred to above is the upward flash which takes place in the form of an upward positive streamer/leader process without the presence of a negative descending leader. The probability of this type of lightning strokes becomes significant in structures with heights in excess of 100 m on flat ground. They can also take place on much shorter structures on mountain tops. Here the field enhancement at and in the proximity of the structure is caused by the induced positive charge on the structure directly caused by the negative charge of the cloud alone since no descending leader is present.
For upward lightning the ambient (ground) field needed for positive leader inception depends mostly on the structure height. For tall structures the critical ambient field is in fact related to the structure height by the simple relationship Eg=1600/h where Eg is given in kV/m and the structure height is given in meters (ref. [1], [2]). Even for the tallest structures the critical ambient field should exceed 3 kV/m (ref. [1], [2]). Therefore and once again if the intent is to reduce the risk of an upward lightning strike, it will be of great advantage for any structure to remain electrically silent, i.e. to be inactive or inhibited or to require higher fields than normal to participate in the game of producing long positive streamers.